Methane conversion process

ABSTRACT

A method for converting methane to higher hydrocarbon products and coproduct water wherein a gas comprising methane and a gaseous oxidant are contacted with a nonacidic catalyst at temperatures within the range of about 700° to 1200° C. in the presence of a halogen promoter, the contacting being conducted in the substantial absence of alkali metals or compounds thereof.

BACKGROUND OF THE INVENTION

This invention relates to the synthesis of hydrocarbons from a methanesource. A particular application of this invention is a method forconverting natural gas to more readily transportable material.

A major source of methane is natural gas. Other sources of methane havebeen considered for fuel supply, e.g., the methane present in coaldeposits or formed during mining operations. Relatively small amounts ofmethane are also produced in various petroleum processes.

The composition of natural gas at the wellhead varies but the majorhydrocarbon present is methane. For example, the methane content ofnatural gas may vary within the range from about 40 to about 95 volumepercent. Other constituents of natural gas include ethane, propane,butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbondioxide, helium and nitrogen.

Natural gas is classified as dry or wet depending upon the amount ofcondensable hydrocarbons contained in it. Condensable hydrocarbonsgenerally comprise C₃ + hydrocarbons although some ethane may beincluded. Gas conditioning is required to alter the composition ofwellhead gas, processing facilities usually being located in or near theproduction fields. Conventional processing of wellhead natural gasyields processed natural gas containing at least a major amount ofmethane.

Large scale use of natural gas often requires a sophisticated andextensive pipeline system. Liquefaction has also been employed as atransportation means, but processes for liquefying, transporting, andrevaporizing natural gas are complex, energy-intensive and requireextensive safety precautions. Transport of natural gas has been acontinuing problem in the exploitation of natural gas resources. Itwould be extremely valuable to be able to convert methane (e.g., naturalgas) to more readily handleable or transportable products. Moreover,direct conversion to olefins such as ethylene or propylene would beextremely valuable to the chemical industry.

U.S. Pat. No. 4,199,533 discloses a process for converting methane tohigher molecular weight hydrocarbons by using chlorine gas as arecyclable catalyst. The process produces ethylene as a major productalong with hydrogen chloride, which is converted to chlorine for recyclein the system. Major drawbacks of the '533 process are the large amountof chlorine required, the necessity of regenerating chlorine fromhydrogen chloride to maintain an economically viable system, and theneed to use operating temperatures in excess of 1000° C. to produceethylene.

Recently, it has been discovered that methane may be converted to higherhydrocarbons (e.g., ethane, ethylene and higher homologs) with minimalformation of carbon oxides by contacting methane with a reducible metaloxide as a selective oxygen source. As the methane is converted tohydrocarbon products and coproduct water, the active oxygen of the metaloxide is depleted, resulting in a reduced metal oxide. The reduced metaloxide is relatively inactive for the oxidative conversion of methane butactive oxygen may be replaced by regeneration of a reducible metaloxide. Such regeneration is accomplished by reoxidation of the reducedmetal oxide.

Reducible oxides of several metals have been identified which arecapable of converting methane to higher hydrocarbons. Oxides ofmanganese, tin, indium, germanium, lead, antimony and bismuth areparticularly useful. See U.S. Pat. Nos. 4,443,649; 4,444,984; 4,443,648;4,443,645; 4,443,647; 4,443,644; and 4,443,646. Also see U.S. Pat. Nos.4,499,323 and 4,499,324.

U.S. Pat. No. 4,499,322 discloses and claims a process for theconversion of methane to higher hydrocarbon which comprises contactingmethane with an oxidative synthesizing agent containing a promotingamount of alkali metal and/or compounds thereof.

U.S. Pat. No. 4,495,374 discloses and claims a process for theconversion of methane to higher hydrocarbons which comprises contactingmethane with an oxidative synthesizing agent containing a promotingamount of alkaline earth metal and/or compounds thereof.

Hinsen and Baerns report studies of a continuous mode for the oxidativecoupling of methane wherein regenerating air is cofed with the methanefeed. Hinsen, W. and Baerns, M., "Oxidative Koppling von Methan zu C₂-Kohlenwasserstoffen in Gegenwart unterschiedlicher Katalysatoren",Chemiker-Zeitung, Vol. 107, No. 718, pp. 223-226 (1983). Using acatalyst based on lead oxide and gamma-alumina in a fixed bed reactoroperated at 1 atmosphere total pressure and 600°-750° C., they reportresults of approximately 53% selectivity to ethane and ethylene at 8%methane conversion for a feed consisting of about 50% methane, 25% airand 25% nitrogen. Other metal oxides studies by Hinsen and Baernsincluded oxides of Bi, Sb, Sn and Mn.

U.S. Pat. No. 4,523,050, filed Apr. 16, 1984, discloses and claims aprocess for converting methane to higher hydrocarbons which comprisescontacting methane and an oxygen-containing gas with a solid comprisinga reducible metal oxide and an alkali/alkaline earth metal promoter.

Commonly-assigned U.S. patent application Ser. No. 600,670, filed Apr.16, 1984, discloses and claims a process for converting methane tohigher hydrocarbons which comprises contacting methane and anoxygen-containing gas with a manganese silicate.

Concurrently-filed, commonly-assigned U.S. patent application Ser. No.738,110 discloses and claims a method for converting methane to higherhydrocarbons wherein methane and a gaseous oxidant are contacted with anonacidic solid. In a preferred embodiment, the solid comprises analkali metal component associated with a support material. Theapplication also teaches conducting the contacting in the presence ofhalogen promoters when employing alkali-promoted solids.

Oxidative condensation of methane in the presence of solid superacidcatalysts is disclosed in U.S. Pat. Nos. 4,433,192; 4,465,893; and4,467,130. European Published Patent Application 93,543 discloses aprocess for aromatizing methane wherein a methane feedstock and anoxidizing agent other than molecular oxygen are contacted attemperatures of about 10° to 600° C. with a solid acidic catalyst havingBronsted acid sites.

The reaction products of such processes are hydrocarbons, carbon oxides,coke and water. It would be beneficial in these processes to reduceselectivities to carbon oxides and coke and to increase methaneconversions to the desired hydrocarbon products. Accordingly, an objectof this invention is to provide an improved process for convertingmethane to higher hydrocarbons. More particular aspects, objects and theseveral advantages of this invention will become apparent to thoseskilled in the art upon reading this disclosure and the appended claims.

SUMMARY OF THE INVENTION

It has now been found that methane may be converted to higherhydrocarbons by contacting a gas comprising methane and a gaseousoxidant with a nonacidic solid at a temperature within the range ofabout 700° to 1200° C. in the presence of a promoter selected from thegroup consisting of halogens and compounds thereof, said contactingbeing conducted in the substantial absence of alkali metals or compoundsthereof. The promoter may be incorporated into the catalyst prior toconducting the contacting, but preferably the promoter is introducedwith methane and gaseous oxidant while conducting the contacting.Halogens are selected from the group consisting of F, Cl, Br and I.Presently preferred promoters are chlorine, bromine and compoundsthereof. Chlorine and compounds of chlorine are particularly preferred.

A desired characteristic of the solid catalyst is that it besubstantially nonreducible under process conditions. While acharacteristic of the present process is coproduction of water, thepresent process does not require the presence of reducible metal oxides.

The catalytic process of this invention offers the advantage ofemploying simpler, less complex solid systems than those processes,described above, which employ solids comprising reducible metal oxides.Moreover, use of nonacidic catalysts minimizes the coking and productdecomposition problems encountered with acidic systems. Furthermore,conducting the contacting in the substantial absence of alkali metalpromoters has been found to enhance the formation of unsaturatedproducts: the ratio of ethylene-to-ethane in the effluent produced bythe present process is higher, thus producing an effluent more amenableto further conversion to normally liquid hydrocarbons.

DETAILED DESCRIPTION OF THE INVENTION

In addition to methane the hydrocarbon feedstock employed in the methodof this invention may contain other hydrocarbon or non-hydrocarboncomponents. The methane content of the feedstock, however, willtypically be within the range of about 40 to 100 vol. %, preferablywithin the range of about 80 to 100 vol. %, more preferably within therange of about 90 to 100 vol. %.

The gaseous oxidant is selected from the group consisting of molecularoxygen, oxides of nitrogen, and mixtures thereof. Preferably, thegaseous oxidant is an oxygen-containing gas. A preferredoxygen-containing gas is air. Suitable oxides of nitrogen include N₂ O,NO, N₂ O₃, N₂ O₅ and NO₂. Nitrous oxide (N₂ O) is a presently preferredoxide of nitrogen.

The ratio of hydrocarbon feedstock to gaseous oxidant gas is notnarrowly critical to the present invention. However, the ratio willdesirably be controlled to avoid the formation of gaseous mixtureswithin the flammable region. The volume ratio of hydrocarbon/gaseousoxidant is preferably within the range of about 0.1-100:1, morepreferably within the range of about 1-50:1. Methane/gaseous oxidantfeed mixtures containing about 50 to 90 volume % methane have been foundto comprise a desirable feedstream.

The solid employed in this invention is generally characterized as"nonacidic". This desciptor is meant to refer to the main, predominantsurface properties of the solids. For example some solid bases are knownto have acidic properties to some extent. See Tanabe, K., "Solid Acidand Base Catalysts." In: Catalysis Science & Technology, vol. 2 (NewYork, Springer - Verlag Berlin Heidelberg, 1981). Currently preferredsolids used in the present process are characterized by negligibleacidity (less than about 0.01 meq/gm) in the H_(o) range less than about3.3, preferably less than about 6.8. H_(o) is the Hammett acidityparameter described on pp. 234-241 of Tanabe.

A further characteristic of preferred solids for the present process isa relatively low surface area. Solids having surface areas less thanabout 50 cm² /gm are suitable, but the surface areas of preferred solidsare within the range of about 0.1-10 m² /gm.

A still further characteristic of preferred solids for the presentprocess is that they be stable and substantially nonreducible underprocess conditions. Examples of suitable solids include those solidbases described in Table 2 on p. 233 of Tanabe, supra. However,presently preferred solids are metal oxides and mixed oxides. Alkalineearth oxides are particularly preferred, especially MgO and CaO. Othersuitable metal oxides are SiO₂, alpha-Al₂ O₃, La₂ O₃, ThO₂, TiO₂, andZrO₂. Such materials are relatively stable under the conditions of thepresent process.

Halogen promoters are preferably introduced into the process withgaseous feedstreams flowing to the process. Any suitable concentrationof promoter can be used. The promoter can be introduced continuously orperiodically, although continuous introduction is preferred. Suitablesources of halogen include free halogen gas, hydrogen halides, ammoniumhalides, aliphatic halides (e.g., methyl halide, methylene halide, ethylhalide, amyl halide, allyl halide), cycloaliphatic halides (e.g.,cyclohexyl halide), halogen substituted aliphatic acids such as methylamine hydrochloride, and the like. Mixtures of various halogen sourcesmay be used. Presently preferred are free halogen gas, aliphatic halidesand hydrogen halides. Methane/gaseous oxidant feed mixtures containingabout 0.01 to 10 vol. % halogen promoter, preferably 0.1 to 5 vol. %,are desirable feedstreams.

Preferably, methane and oxygen are contacted with the agent in thesubstantial absence of catalytically effective nickel, noble metals andcompounds thereof. (i.e., nickel, rhodium, palladium, silver, osmium,iridium, platinum and gold) to minimize the deleterious catalyticeffects thereof. These metals, when contacted with methane at thetemperatures employed in the first step of the present invention, tendto promote coke formation, and the metal oxides tend to promote theformation of combustion products rather than the desired hydrocarbons.The term "catalytically effective" is used herein to identify thatquantity of one or more of nickel and of the noble metals and compoundsthereof which substantially changes the distribution of productsobtained in the method of this invention relative to such contacting inthe absence of such metals and compounds thereof.

Operating temperatures for the method of this invention are generallywithin the range of about 700° to 1200° C., more preferably within therange of about 800° to 1000° C.

Operating pressures are not critical to the presently claimed invention.However, both general system pressure and partial pressures of methaneand oxygen have been found to effect overall results. Preferredoperating pressures are within the range of about 0.1 to 30 atmospheres.

The space velocity of the gaseous reaction streams are similarly notcritical to the presently claimed invention, but have been found toeffect overall results. Preferred total gas hourly space velocities arewithin the range of about 10 to 100,000 hr.⁻¹, more preferably withinthe range of about 600 to 40,000 hr.⁻¹.

The catalyst may be maintained in the contact area as fixed, moving orfluidized beds of solids. A fixed bed of solids is currently preferredfor the method of this invention.

The catalyst may be maintained in the contact zone as fixed, moving afluidized beds of solids. A fixed bed of solids is currently preferredfor the method of this invention.

The effluent from the contact zone contains higher hydrocarbon products(e.g., ethylene, ethane and other light hydrocarbons), carbon oxides,water, unreacted hydrocarbon (e.g., methane) and oxygen, and other gasespresent in gas fed to the contact zone. Higher hydrocarbons may berecovered from the effluent and, if desired, subjected to furtherprocessing using techniques known to those skilled in the art. Unreactedmethane may be recovered and recycled to the contact zone.

The invention is further illustrated by reference to the followingexamples. Experimental results reported below include conversions andselectivities calculated on a carbon mole basis. Ethylene/ethane ratiosare shown as molar ratios.

EXAMPLE 1

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining varying amounts of HCl was contacted with MgO (supplied byKaiser Chemicals) and the results shown in Table I were obtained.

                  TABLE 1                                                         ______________________________________                                        CH.sub.4 /Air/HCl over MgO                                                    Vol %                                                                         HCl            CH.sub.4                                                       in     Temp    GHSV     % CH.sub.4                                                                          % C.sub.2 +                                     feed   (°C.)                                                                          (hr..sup.-1)                                                                           Conv. Selectivity                                                                            C.sub.2 =/C.sub.2                      ______________________________________                                        0      775     2400     16.0  33.1     0.56                                   0.05   775     2400     20.0  42.3     1.17                                   0.5    775     2400     29.6  68.9     6.14                                   1.0    775     2400     23.5  65.1     7.39                                   0      825     2400     16.9  42.2     0.96                                   0.05   825     2400     20.3  52.9     1.66                                   0.5    825     2400     29.3  71.9     5.50                                   1.0    825     2400     28.1  71.4     11.46                                  0      900     2400     20.6  48.9     2.57                                   0.5    900     2400     29.9  67.5     10.37                                  1.0    900     2400     26.9  72.0     20.75                                  0      800     4800     14.7  31.9     0.56                                   0.5    800     4800     29.1  69.8     5.00                                   1.0    800     4800     29.2  65.1     8.66                                   0      900     4800     19.9  55.2     1.39                                   0.5    900     4800     29.9  71.5     5.77                                   ______________________________________                                    

EXAMPLE 2

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining 0.5 vol. % CHF₃ was contacted with MgO (supplied by KaiserChemicals) and the results shown in Table 2 were obtained.

                  TABLE 2                                                         ______________________________________                                        CH.sub.4 /Air/CHF.sub.3 Over MgO                                                     CH.sub.4                                                               Temp   GHSV     % CH.sub.4                                                                             % C.sub.2 +                                          (°C.)                                                                         (hr.sup.-1)                                                                            Conv.    Selectivity                                                                            C.sub.2 =/C.sub.2 Ratio                     ______________________________________                                        700    2400     2.9      35.5     0.1                                         750    2400     3.8      39.6     0.2                                         775    2400     5.9      50.9     0.4                                         825    2400     9.0      60.5     0.7                                         700    4800     2.5      85.0     3.1                                         750    4800     1.9      48.7     0.1                                         800    4800     3.3      64.7     0.6                                         900    4800     8.9      65.8     0.9                                         ______________________________________                                    

EXAMPLE 3

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining 0.5 vol. % HCl was contacted with MgO (supplied by CatalystResources, Inc.) and the results shown in Table 3 were obtained.

                  TABLE 3                                                         ______________________________________                                        CH.sub.4 /Air/HCl Over MgO                                                           CH.sub.4                                                               Temp   GHSV     % CH.sub.4                                                                             % C.sub.2 +                                          (°C.)                                                                         (hr.sup.-1)                                                                            Conv.    Selectivity                                                                            C.sub.2 =/C.sub.2 Ratio                     ______________________________________                                        775    2400     11.5     77.3     2.80                                        825    2400     20.7     64.8     7.05                                        900    2400     27.1     71.8     15.05                                       700    4800     0.8      88.6     0.16                                        750    4800     1.5      90.7     0.50                                        800    4800     6.3      90.8     1.59                                        900    4800     26.0     69.5     7.67                                        ______________________________________                                    

EXAMPLE 4

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining 0.5 vol. % HCl was contacted with CaO and the results shownin Table 4 were obtained.

                  TABLE 4                                                         ______________________________________                                        CH.sub.4 /Air/HCl Over CaO                                                           CH.sub.4                                                               Temp   GHSV     % CH.sub.4                                                                             % C.sub.2 +                                          (°C.)                                                                         (hr.sup.-1)                                                                            Conv.    Selectivity                                                                            C.sub.2 =/C.sub.2 Ratio                     ______________________________________                                        700    2400     6.6      26.4     0.4                                         750    2400     15.1     35.9     0.7                                         775    2400     13.6     43.5     0.7                                         800    2400     18.7     46.7     1.1                                         900    2400     18.8     56.5     --                                          800    4800     15.6     62.2     2.7                                         900    4800     24.2     71.4     6.5                                         ______________________________________                                    

EXAMPLE 5

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining varying amounts of HCl was contacted with La₂ O₃ and theresults shown in Table 5 were obtained.

                                      TABLE 5                                     __________________________________________________________________________    CH.sub.4 /Air/HCl Over La.sub.2 O.sub.3                                       Vol %                                                                         HCl      CH.sub.4                                                             in   Temp                                                                              GHSV % Conv.                                                                             % Selectivity to:                                         feed (°C.)                                                                      (hr..sup.-1)                                                                       CH.sub.4                                                                         O.sub.2                                                                          C.sub.2 =                                                                        C.sub.2                                                                           C.sub.2 +                                                                        CO CO.sub.2                                     __________________________________________________________________________    0.58 700 2400 18.5                                                                             88.4                                                                             6.8                                                                              16.3                                                                              24.2                                                                             39.9                                                                             35.9                                         0.58 750 2400 22.0                                                                             92.7                                                                             14.5                                                                             19.5                                                                              35.8                                                                             33.2                                                                             31.0                                         0.58 775 2400 22.1                                                                             93.4                                                                             18.8                                                                             20.3                                                                              41.3                                                                             29.2                                                                             29.5                                         0.58 800 2400 24.2                                                                             87.5                                                                             26.4                                                                             22.1                                                                              51.7                                                                             19.8                                                                             28.5                                         0.58 900 2400 24.6                                                                             93.0                                                                             36.8                                                                             16.1                                                                              57.0                                                                             19.1                                                                             23.9                                         0.5  700 4800 5.7                                                                              14.8                                                                             9.8                                                                              16.0                                                                              26.1                                                                             47.5                                                                             26.5                                         0.5  750 4800 8.8                                                                              22.5                                                                             22.4                                                                             17.1                                                                              40.9                                                                             38.8                                                                             20.3                                         0.5  800 4800 14.1                                                                             38.9                                                                             34.8                                                                             12.9                                                                              50.1                                                                             34.8                                                                             15.1                                         0.5  900 4800 23.8                                                                             78.1                                                                             41.2                                                                             7.1 51.7                                                                             36.4                                                                             11.9                                         __________________________________________________________________________

EXAMPLE 6

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining 0.5 vol. % HCl was contacted with TiO₂ (anatase) and theresults shown in Table 6 were obtained.

                  TABLE 6                                                         ______________________________________                                        CH.sub.4 /Air/HCl Over TiO.sub.2                                                     CH.sub.4                                                               Temp   GHSV     % CH.sub.4                                                                             % C.sub.2 +                                          (°C.)                                                                         (hr.sup.-1)                                                                            Conv.    Selectivity                                                                            C.sub.2 =/C.sub.2 Ratio                     ______________________________________                                        750    2400     9.4      48.2     1.6                                         775    2400     14.2     45.3     2.6                                         825    2400     19.6     40.8     3.9                                         900    2400     20.2     32.0     3.2                                         700    4800     2.0      89.0     0.6                                         750    4800     4.0      92.7     1.3                                         800    4800     13.0     49.7     2.5                                         900    4800     18.8     36.5     2.9                                         ______________________________________                                    

EXAMPLE 7

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining 0.5 vol. % HCL was contacted with SiO₂ and the results shownin Table 7 were obtained.

                  TABLE 7                                                         ______________________________________                                        CH.sub.4 /Air Over SiO.sub.2                                                         CH.sub.4                                                               Temp   GHSV     % CH.sub.4                                                                             % C.sub.2 +                                          (°C.)                                                                         (hr.sup.-1)                                                                            Conv.    Selectivity                                                                            C.sub.2 =/C.sub.2 Ratio                     ______________________________________                                        775    2400     1.7      94.7     1.52                                        825    2400     4.5      58.0     3.01                                        800    4800     0.66     100      0.93                                        900    4800     7.4      56.4     3.74                                        ______________________________________                                    

EXAMPLE 8

A gaseous feedstream consisting of 50 vol. % air in methane andcontaining 0.5 vol. % HCL was contacted with alpha-Al₂ O₃ and theresults shown in Table 8 were obtained.

                  TABLE 8                                                         ______________________________________                                        CH.sub.4 /Air/HCl Over α-Al.sub.2 O.sub.3                                      CH.sub.4                                                               Temp   GHSV     % CH.sub.4                                                                             % C.sub.2 +                                          (°C.)                                                                         (hr.sup.-1)                                                                            Conv.    Selectivity                                                                            C.sub.2 =/C.sub.2 Ratio                     ______________________________________                                        700    2400     0.81     61.2     0.4                                         750    2400     2.8      39.7     0.7                                         775    2400     1.1      83.7     0.7                                         825    2400     1.7      88.6     1.4                                         900    2400     9.8      53.0     4.7                                         700    4800     0.28     63.1     0.3                                         750    4800     0.25     70.4     0.3                                         800    4800     0.43     83.1     0.5                                         900    4800     2.1      93.5     1.5                                         ______________________________________                                    

What is claimed is:
 1. A method for converting methane to higherhydrocarbon products and coproduct water which comprises contacting agas comprising methane and a gaseous oxidant with solid La₂ O₃ at atemperature within the range of about 700° to 1200° C. in the presenceof at least one gaseous promoter selected from the group consisting ofhalogens and compounds thereof, said contacting being conducted in thesubstantial absence of alkali metals and compounds thereof and in thesubstantial absence of reducible metal oxides.
 2. The method of claim 1wherein the promoter is selected from the group consisting of chlorineand mixtures thereof.
 3. The method of claim 2 wherein the promoter isHCl.
 4. The method of claim 2 wherein the promoter is an aliphaticchloride.
 5. The method of claim 2 wherein the promoter is Cl₂.
 6. Themethod of claim 1 wherein the promoter is selected from the groupconsisting of bromine and compounds thereof.
 7. The method of claim 6wherein the promoter is HBr.
 8. The method of claim 6 wherein thepromoter is an aliphatic bromide.
 9. The method of claim 6 wherein thepromoter is Br₂.
 10. The method of claim 1 wherein the gaseous oxidantcomprises molecular oxygen.
 11. The method of claim 1 wherein thegaseous oxidant comprises oxides of nitrogen.
 12. The method of claim 11wherein the oxides of nitrogen comprise N₂ O.
 13. The method of claim 1wherein the temperature is within the range of about 800° to 1000° C.